1. Field of the Invention
The present invention relates to a stencil printing machine having a cylindrical printing drum which has a stencil sheet wound around an outer peripheral surface thereof and is rotatably driven.
2. Description of the Related Art
FIG. 16 is a cross-sectional view showing a basic structure of a conventional stencil printing machine.
The stencil printing machine includes a printing drum 100. The printing drum 100 has a base body 103 which is comprised of a pair of circular plate members 101 arranged coaxially on a common central axis and a stencil clamping base 102 which connects a pair of circular plate members 101. A gear 104 is fixedly and coaxially mounted on the base body 103. The gear 104 is meshed with a drive gear not shown in the drawing. Upon rotation of the drive gear, the base body 103 is rotatably driven about the central axis.
An ink passing and flexible ink passing member 105 is wound around the base body 103. The ink passing member 105 has one end thereof fixedly secured to the stencil clamping base 102 of the base body 103. The ink passing member 105 is wound around outer peripheral surfaces of the circular plate members 101. The ink passing member 105 has the other end thereof elastically mounted on the stencil clamping base 102 by means of a spring 106. Accordingly, when the ink passing member 105 is pushed from the inside, the ink passing member 105 is elastically deformed outwardly. The stencil clamping base 102 is provided with clamp means 107. A perforated stencil sheet has a leading end thereof fixedly secured to the stencil clamping base 102 by means of clamp means 107 and is wound around an outer peripheral surface of the ink passing member 105.
In the inside of the printing drum 100, a squeezing roller 110 is elevatably mounted. In synchronous with the rotation of the printing drum 100, the squeezing roller 110 is elevated at a given timing and presses the ink passing member 105 outwardly. The squeezing roller 110 is rotatably driven in the same direction as the printing drum 100 in an interlocking manner with the printing drum 100 by means of the drive gear 104 of the printing drum 100, an intermediate gear 111 and a gear 112 of the squeezing roller 110. Ink is supplied to the inside of the printing drum 100 by way of an ink supply pipe 113 and a constant amount of ink is supplied to a peripheral surface of the squeezing roller 110 by means of a doctor roller 114 which is arranged against the squeezing roller 110 at a given interval.
Although not shown in the drawing, a pressure drum having a diameter approximately equal to that of the printing drum 100 is disposed below the printing drum 100. The axis of rotation of the pressure drum is parallel to the central axis of the printing drum 100. In the non-printing state, a minute clearance is formed between the printing drum 100 and the pressure drum. The pressure drum is rotatably driven in synchronous with the rotation of the printing drum 100 but in the opposite direction. A printing paper is fed between the printing drum 100 and the pressure drum. The squeezing roller 110 is lowered at a given timing so as to push the ink passing member 105 outwardly. The ink passing member 105 which is deformed outwardly and a stencil sheet clamp the printing paper between these members and the pressure drum. The printing paper is conveyed while being clamped between the deformed printing drum 100 and the pressure drum. During the conveyance, ink supplied to the inner peripheral surface of the ink passing member 105 is transferred to the printing paper through the ink passing member 105 and a perforated portion of the stencil sheet thus forming an image.
An image of an original to be printed is read by an image reader 200 and is supplied to a stencil making section 300 as image signals.
The stencil making section 300 includes a platen roller 301 and a thermal head 302. The stencil sheet M having a roll shape is clamped between the platen roller 301 and the thermal head 302 and is conveyed. During conveyance, the thermal head 302 is driven by an image signal fed from the image reader 200 and generates heat and forms a perforated image in the stencil sheet M. The stencil sheet M in which the perforated image is formed is conveyed to a position in the vicinity of the printing drum 100 by means of the platen roller 301 and feed rollers 303. The stencil sheet M has a leading end portion thereof fixedly secured onto the stencil clamping base 102 by means of the clamp means 107 and is wound around the outer peripheral surface of the printing drum 100 along with the rotation of the printing drum 100 in the counterclockwise direction in the drawing. When the stencil sheet M is wound around the printing drum 100 by a given dimension corresponding to one sheet, the stencil sheet M is cut by a cutter 304. This cut portion disposed at the side opposite to the leading end portion of the stencil sheet M which is fixedly secured to the clamp means 107 constitutes a trailing end portion of the stencil sheet M.
Thereafter, the printing is performed. After printing, the used stencil sheet M is peeled off from the printing drum 100 and is removed. With the clamp means 107 in the opened state, the printing drum 100 is rotated. The leading end portion of the stencil sheet M released from the clamp means 107 is clamped between a pair of stencil discharge rollers 401 and is stacked in a stencil discharge section 400 including a stencil discharge box by driving the stencil discharge rollers 401. At the time of discharging the used stencil sheet M, ink remaining on the used stencil sheet M is adhered to the stencil discharge rollers 401.
In the above-mentioned state, when the stencil making and the printing are performed based on a new image, at the time of winding the newly perforated stencil sheet M which is fed from the feed rollers 303 around the printing drum 100 and printing, there has been a trouble that the trailing end portion which is not fixedly secured by the clamp means 107 is lifted and comes into contact with the stencil discharge roller 401 or the like so that ink is adhered to it. Further, it gives rise to several problems including a problem that ink adhered to the trailing end portion of the stencil sheet M is transferred to the printing member thus staining the printing paper.
Further, in the stencil printing machine, in case of performing a duplex printing, as shown in FIG. 17, for example, it is considered to prepare two rotatable cylindrical printing drums 501, 502 which have outer peripheral surfaces thereof wound around by stencil sheets M. These printing drums 501, 502 have axes of rotation which are disposed parallel to each other. Further, in the insides of respective printing drums 501, 502, squeezing rollers 503, 504 which supply ink to the outer peripheries of the printing drums 501, 502 are disposed. Subsequently, a printing paper P is fed between respective printing drums 501, 502 and then the printing paper P is nipped between the outer peripheral surfaces of the printing drums 501, 502 so as to transfer the ink which passes through the stencil sheets M onto the printing paper P thus completing the duplex printing.
Further, for winding the stencil sheets M around the outer peripheral surfaces of the printing drums, the printing drums 501, 502 are respectively provided with stopper mechanisms for clamping leading end portions of the conveyed stencil sheets M to portions of the outer peripheral surfaces of the printing drums 501, 502. These stopper mechanisms include clamp plates 505, 506 which can be opened and closed. The clamp plates 505, 506 clamp the leading end portions of the stencil sheets M by closing operations and release the leading end portions of the stencil sheets M by opening operations.
Further, the above-mentioned stopper mechanisms respectively clamp the leading end portions of the stencil sheets M such that the leading end portions of the stencil sheets M are directed toward the rotational directions of the printing drums 501, 502. Then, in the conventional stencil printing machine, respective printing drums 501, 502 have the same diameter and are rotated at the same peripheral speed. That is, in the conventional stencil printing machine, when the printing drums 501, 502 are rotated, the stencil sheets M are brought into contact with the printing paper P and receive loads and hence, the leading end portions of the stencil sheets M are uniformly pulled.
However, in the above-mentioned conventional stencil printing machine, although respective printing drums 501, 502 are designed such that they have the same diameter to be rotated at the same peripheral speed theoretically, to observe the entire periphery of the printing drum 501 (502) locally, there exists a case that their diameters are slightly different from each other. In this manner, when there exists the difference in diameter between respective printing drums 501, 502, the line speeds of respective stencil sheets M wound around respective printing drums 501, 502 become different from each other and hence, stresses which make respective stencil sheets M move in the winding direction work on respective stencil sheets M.
To be more specific, when the diameter of the upper-side printing drum 501 is made large, the line speed of the stencil sheet M wound around the printing drum 501 is increased, while the line speed of the stencil sheet M wound around the lower-side printing drum 502 having the small diameter is decreased.
Then, as shown in FIG. 18, the printing paper P is brought into contact with respective stencil sheets M which differ in line speed. In this case, although the stencil sheet M of the upper-side printing drum 501 receives a stress F1 which resists the rotational direction of the printing drum 501, since the leading end portion is clamped by the stopper mechanism, the stencil sheet M is not moved. To the contrary, since the stencil sheet M of the lower-side printing drum 502 receives a stress F2 which follows the rotational direction of the printing drum 502, the stencil sheet M is moved in the direction of the leading end portion which is clamped by the stopper mechanism so that the stencil sheet M is displaced from its original position. Accordingly, there have been several problems including following problems. That is, a printed image formed by the stencil sheet M wound around the lower-side printing drum 502 having the low line speed is shrunk or contracted or wrinkles are formed on the stencil sheet M. Since the stencil sheet M moves to a position where the stencil sheet M cannot cover a printing region (a region which allows ink to pass through) of the printing drum 502, the printing paper P is stained.
Accordingly, it is an object of the present invention to provide a stencil printing machine which can prevent ink from staining a trailing end portion of a stencil sheet and accordingly preventing such ink from adhering to and staining a printing paper.
It is another object of the present invention to provide a stencil printing machine which can prevent the movement of stencil sheets wound around respective printing drums when a printing is performed on both surfaces of a printing sheet between a plurality of printing drums simultaneously.
According to the first aspect of the present invention, there is provided a stencil printing machine comprising a printing drum 1 which has a perforated stencil sheet M wound around an outer peripheral surface thereof and is rotatable about a central axis thereof, first holding means 10 which is provided to the outer peripheral surface of the printing drum and holds a leading end of the stencil sheet, and second holding means 18 which is provided to the outer peripheral surface of the printing drum and holds a trailing end of the stencil sheet.
According to the second aspect of the present invention, in the stencil printing machine of the first aspect of the present invention, a fan 32 is provided at a position in the vicinity of the printing drum 1 and the fan 32 blows wind to the trailing end of the stencil sheet M wound around the outer peripheral surface of the printing drum so as to move the trailing end toward the outer peripheral surface of the printing drum thereby enabling the second holding means 18 to hold the trailing end between the second holding means 18 and the printing drum.
According to the third aspect of the present invention, in the stencil printing machine of the first aspect of the present invention, a press member (a press roller 60) is movably provided at a position in the vicinity of the printing drum 1 and the press member presses the trailing end of the stencil sheet M wound around the outer peripheral surface of the printing drum onto the outer peripheral surface of the printing drum thereby enabling the second holding means 18 to hold the trailing end between the second holding means 18 and the printing drum.
According to the fourth aspect of the present invention, there is provided a stencil printing machine including a base body 3 which is comprised of a pair of circular plate members 4 which are arranged coaxially on a common central axis and a first base member (a first stencil clamping base 5) which connects a pair of the circular plate members. The base body 3 is rotatable about the above-mentioned central axis. Further, the stencil printing machine includes an ink passing member 15 which has one end portion thereof mounted on the base body, is wound around a pair of circular plate members and forms a peripheral surface for mounting a stencil sheet M thereon. Further, the stencil printing machine includes a second base member (a second stencil clamping base 16) which is mounted on the other end of the ink passing member, an elastic member (a spring 17) which connects the second base member to the base body such that the ink passing member is wound around a pair of circular plate members with a given force and first holding means 10 which is mounted on the base body for holding a leading end of a stencil sheet M, and second holding means 18 which is mounted on the second base body for holding a trailing end of the stencil sheet M.
According to the fifth aspect of the present invention, there is provided a stencil printing machine including at least two printing drums 1, 2 which wind perforated stencil sheets on outer peripheral surfaces thereof and are rotatable about central axes thereof, first holding means 10, 10 which are provided to outer peripheral surfaces of the respective printing drums 1, 2 for holding leading end portions of the stencil sheets M, M, and second holding means 18, 18 which are provided to outer peripheral surfaces of the respective printing drums 1, 2 for holding trailing end portions of the stencil sheets M, M.
According to the sixth aspect of the present invention, in the stencil printing machine of the fifth aspect of the present invention, the peripheral speed of one of the printing drums 1(2) is set slower than the peripheral speed of the other printing drum 2(1).
According to the seventh aspect of the present invention, in the stencil printing machine of the sixth aspect of the present invention, the peripheral speeds of respective printing drums 1, 2 are set by changing the diameters of outer peripheral surfaces of the respective printing drums 1, 2.
According to the eighth aspect of the present invention, in the stencil printing machine of any one of fifth to seventh aspects of the present invention, fans 32, 32 are provided at positions in the vicinity of the respective printing drums 1, 2 and the fan 32, 32 blow wind to the respective trailing end portions of the stencil sheets M, M wound around the outer peripheral surfaces of the printing drums 1, 2 so as to move the trailing end portions toward the outer peripheral surfaces of the printing drums thereby enabling the second holding means 18, 18 to hold the trailing end portions between the second holding means 18, 18 and the printing drums 1, 2.
According to the ninth aspect of the present invention, in the stencil printing machine of any one of fifth to seventh aspects of the present invention, press members (press rollers 60, 60) are movably provided at positions in the vicinity of the printing drums 1,2 and the press members press the respective trailing end portions of the stencil sheets M, M wound around the outer peripheral surfaces of the printing drums 1, 2 onto the outer peripheral surfaces of the printing drums 1, 2 thereby enabling the second holding means 18, 18 to hold the trailing end portions between the second holding means 18, 18 and the printing drums 1, 2.